(Supported by NSF DMB 8916315 to C.A. Mannella) Cubic membranes are symmetric, periodic structures that occur in a wide variety of living systems. There are several varieties of crystal structure proposed for the observed membrane arrays in different biological systems. In previous work (Deng, et al. Protozoology, in press), a template matching method was developed to indirectly identify the crystal type, using electron microscopy of amoeba mitochondria. In this work, we determined the crystal type from IVEM tomographic reconstructions, in comparison to a computer-generated model volume of a perfect crystal of the appropriate type. This validated the template-matching method. We made reconstructions of mitochondria representing examples of two different crystal types, the double-diamond and the gyroid. Extensive use of computer visualization and modeling methods enabled us to not only determine the crystal type but also to understand how the cubic membranes are integrated into the architecture of the mitochondrion, giving insights into their function. In the two examples, the spaces in the cubic area were shown to be intracristal space, which was connected to the space between the inner and outer mitochondrial membranes by tubular connecting regions, as seen in the other types of mitochondria we have studied (see CS subproject "Electron tomography of mitochondria: membrane interactions"). The matrix material was narrowly confined between the membranes in the cubic phase, thus reversing the usual situation in which most of the volume inside the mitochondria is matrix material. Dr. Deng completed her Ph.D. in Buffalo, then came to Albany as a postdoc in Dr. Mannella's lab, where she continued her structural studies. Her next priority will be functional studies on the cubic membranes. The work was formally presented at two meetings and also shown in connection with general talks on mitochondrial structure by Dr. Mannella, at several venues. A manuscript is ready for submission. Deng, Y., Mieczkowski, M., Marko, M., Buttle, K., Rath, B.K., Mannella, C.A. (1998) 3-D structure of periodic cubic-phase inner membranes in mitochondria of the amoebae Chaos carolinensis. Microscopy and Microanalysis vol 4, suppl. 2 (Proc. Microscopy and Microanalysis '98) Ed. G. Bailey, et al., Springer , New York, pp. 432-33. Deng, Y., Marko, M., Buttle, K., Leith, A., Mannella, C.A. (1998) Mitochondria in Chaos. Albany Conference on Frontiers of Mitochondrial Research. Deng, Y., Marko, M., Buttle, K., Mannella, C.A. (1998) Cubic mitochondrial membranes of the amoebae Chaos carolinsis studied by electron tomographic reconstruction. J. Structural Biol. (in preparation).